Sorted, finally.
After having the engine light pop on constantly, checked all the Jube clips on the pipework etc etc, and after reading CJ and Worzles post about air leaks etc etc. Decided to have a real long check. Changed some of the vacuum pipes, then found this...
Ops...
Pipe was hard like toffee, and would just snap when touched. Replaced it with some red pipe ( of course
) took her for a quick blip, and slap me silly with a wet haddock!!! Problem gone.
I only really noticed the full extent of the issue at 2.30am zipping down the A21, gave it some beans ( normally spend my journey to and from work in light/heavy traffic so cant ) and it boosted to 15 psi, spluttered, juddered, then felt like it was braking.
I guess the heat from the Turbo has cooked the pipe over time?